Heretofore, laminated coil components using ferrite-based ceramics such as Ni—Zn having a spinel type crystal structure, are widely used, and ferrite materials are also actively developed.
This kind of laminated coil component has a structure in which a conductor part wound into a coil shape is embedded in a magnetic body part, and usually the conductor part and the magnetic body part are formed by simultaneous firing.
In the above laminated coil component, since the magnetic body part made of a ferrite material has a coefficient of linear expansion different from that of the conductor part containing a conductive material as a main component, stress-strain caused by the difference in the coefficient of linear expansion is internally produced during the process of cooling after firing. When a rapid change in temperature is produced or external stress is loaded due to reflow treatment in mounting a component on a substrate or the like, the above-mentioned stress-strain varies, and therefore magnetic characteristics such as inductance fluctuate.
Then, Japanese Unexamined Utility Model Application Publication No. 6-45307 (Patent Document 1) (see, claim 2, paragraph 0024, FIG. 2, and FIG. 7) proposes a laminated chip inductor in which a framework of a laminated chip is formed by laminated ceramic sheets, a coil conductor is formed in the laminated chip by an internal conductor, and a start end and a terminal end of the coil conductor are separately connected to external electrode terminals, and in which the ceramic sheet is a magnetic sheet, and a doughnut-shaped non-magnetic region is formed in the laminated chip so as to embrace the internal conductor excluding extraction parts to the external electrode terminals.
In this Patent Document 1, after preparing the magnetic sheet, a non-magnetic paste is applied onto the magnetic sheet to form a non-magnetic film with a predetermined pattern, and thereafter, a printing treatment is performed in turn plural times using a magnetic paste, a paste for an internal conductor and a non-magnetic paste, and thereby, a laminated chip inductor is obtained.
In Patent Document 1, by employing a non-magnetic paste for the ceramic in contact with the coil conductor, the magnetic characteristics are prevented from fluctuating even when the stress-strain is internally produced by simultaneous firing and thereafter thermal shock is given or external stress is loaded.
On the other hand, in this kind of a laminated coil component, it is important that stable inductance is attained even when a large current is applied, and it is necessary to this end to have such a DC superposition characteristic that a reduction in inductance is suppressed even when a large DC current is applied.
However, since the laminated coil components such as a laminated inductor form a closed magnetic circuit, magnetic saturation is easily generated to decrease the inductance when a large current is applied, and desired DC superposition characteristics cannot be attained.
Hence, Japanese Patent No. 2694757 (Patent Document 2) (see, claim 1, FIG. 1, etc.) proposes a laminated coil component provided with a conductor pattern having an end connected between magnetic body layers and wound in a direction of lamination in the form of superimposition, and provided with layers of a material having lower magnetic permeability than the magnetic body layer, which are in contact with conductor patterns of both ends in the direction of lamination and located on the inside of the conductor patterns.
In Patent Document 2, by disposing a layer made of a material (for example, a Ni—Fe-based ferrite material having a small Ni content, or a non-magnetic material) having lower magnetic permeability than the magnetic body layer on the outside of the conductor pattern, a magnetic flux is prevented from concentrating at a corner on the inside of the conductor pattern at an end, and the magnetic flux is dispersed toward the center of the main magnetic path, and thereby, the occurrence of magnetic saturation is prevented to improve inductance.
Further, Japanese Patent Laid-open Publication No. 2006-237438 (Patent Document 3) (see, claim 1, paragraph 0007) proposes a laminated bead in which a magnetic body layer and a conductor pattern are laminated, and an impedance element is formed in a base, wherein a sintering modifier for adjusting the sinterability of the magnetic body layer is mixed in a conductive paste.
In Patent Document 3, the sintering modifier is composed of SiO2 with which a silver powder is coated, SiO2 contains silver in an amount of 0.05 to 0.3 wt %, and the conductive paste including the mixed sintering modifier is printed on a magnetic body layer to form a conductor pattern.
Further, in Patent Document 3, by mixing the sintering modifier in the conductive paste, since the sintering modifier is moderately diffused in the magnetic body, it is possible to delay the progress of sintering of the magnetic body near the conductor pattern compared with other portions, and thereby, a magnetically inactive layer is formed in a manner of functional gradient. That is, by delaying the progress of sintering of the magnetic body near the conductor pattern compared with other portions, the grain size of the magnetic body between the conductor patterns or near the conductor pattern becomes smaller than that in other portions to enable formation of a low-magnetic permeability layer, and a magnetically inactive portion is formed. Thereby, it is intended to improve the DC superposition characteristics to a large current region in a high-frequency band to prevent the deterioration of magnetic characteristics.